As the rapid growth of transmission speed in optical communication systems, advanced modulation formats such as Duo-Binary, Carrier Suppressed Return to Zero (CSRZ), Differential Phase Shift Keying (DPSK) and Differential Quadrature Phase Shift Keying (DQPSK), have attracted a wide attention and have been gradually adopted in the systems. In optical DQPSK modulation, two bits of the incoming data are modulated on the carrier by shifting the phase for a multiple of π/2 in a time interval T (also called a symbol). Four possible shifts between the predecessor and successor symbols are determined by four possible combinations of the two bits data. In each symbol, information of two bits is transmitted. Therefore the spectrum efficiency is doubled, the device bandwidth is halved, and the tolerances of chromatic dispersion (CD) and polarization mode dispersion (PMD) are relaxed. DQPSK is considered as one of the most promising modulation formats for the future high speed optical communication systems.
In an optical DQPSK modulator, two Mach-Zender Modulators are incorporated in parallel to shift the phase of the optical carrier. Two driving signals are amplified to 2Vπ and biased at the transmission null. With an additional phase shift of π/2 in the quadrature (Q) branch, the recombined optical signal of the in-phase (I) branch and Q branch is modulated to four possible phase stages. Because of the differential nature of the DQPSK modulation, a pre-encoder is required to provide a direct map between random data and the driving signals. In a conventional pre-encoder, the output signals are fed back to complete a logic decision, which limits the bit rate due to the electronic propagation delay. The implementation of a simple and feasible pre-encoder is of vital importance in the commercial application of optical DQPSK.
U.S. Pat. No. 7,327,961B2 discloses a technique which achieves a higher speed by multiplexing several low-speed pre-encoders in parallel. Each of the low-speed pre-encoder is implemented according to the conventional circuit arrangement with the feedback loop. The multiplexing relaxes the delay restriction to some extent, but introduces additional concerns such as data synchronization.
It's also proposed a circuit arrangement for a serial optical DPQSK modulator in an international application WO2005069490A1. One Mach-Zender Modulator and one Phase Modulator are cascaded comprised to shift the phase of the optical carrier. The Phase Modulator shifts the carrier phase by either π/4 or 0, which requires precise amplitude control of the driving signal. In practical implementation, the serial modulation scheme is less adopted compared with parallel modulation.